Make FQDN lookup function more testable

[ycombinator]

This PR refactors the shared.FQDN() function, extracting the lookup algorithm into a helper function, to make the code a bit more testable. Having done that, the PR also adds a unit test for the helper function.

[elasticmachine]

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• Start Time: 2023-03-27T22:42:51.338+0000

• Duration: 10 min 28 sec

Test stats 🧪

Test	Results
Failed	0
Passed	280
Skipped	4
Total	284

[cmacknz]

Will temporary DNS errors cause this test to fail even if the logic is correct?

[ycombinator]

Yeah, it will. Just tested it with my Internet connection turned off.

I'm hoping such network/DNS unavailability errors are extremely rare, though. Perhaps we could add a comment here and remove this test case if it starts proving to be flaky?

[cmacknz]

I think a comment is a good idea, it won't necessarily be obvious that this requires an internet connection without context.

I'm fine waiting and seeing if this turns out to be flaky. Anyone testing this code will need to hit the real code path anyway so mocking it out doesn't necessarily help.

[ycombinator]

The tests added in this PR are failing in CI with a slightly different error (as returned by shared.fqdn()) than on my development machine. So I'm going to relax the error assertions in the test slightly to make the tests less sensitive to the environment they're running in.

[ycombinator]

/test